Atrazine (ATZ) is an endocrine disruptor that increases aromatase activity. In viviparous (live-bearing) vertebrates, embryos may be exposed to endocrine disruptors via the placenta. Studies of the effects of ATZ in viviparous amniotes have focused on rodents, which are relatively insensitive to ATZ: studies on other viviparous amniotes are therefore required. We aimed to determine the effects of gestational exposure to a single dose of ATZ at 10 ppb on gonadal development in a viviparous skink, Niveoscincus metallicus. Pregnant skinks were exposed to ATZ, the synthetic estrogen diethylstilbestrol (DES) (positive control), vehicle solvent or no treatment. Gonads were examined histologically at birth. Females born to ATZ and DES exposed mothers were more likely to exhibit ovaries with abnormal oocytes than were females whose mothers received vehicle solvent or no treatment. Males born to ATZ and DES exposed mothers were equally more likely to exhibit testes devoid of germ cells with reduced organization of seminiferous tubules (ST) compared to males born to mothers receiving vehicle solvent or no treatment. However, ATZ treatment significantly increased the number of male neonates born with testicular lesions compared to males born to mothers in any other group. We conclude that atrazine disrupts gonadal differentiation in the viviparous lizard, N. metallicus. The similar effects of DES and ATZ suggest that the developmental effects of ATZ in N. metallicus reflect increased estrogen signaling. Atrazine should be used with caution as exposure of wildlife to this EDC is likely to have adverse effects on reproductive health.